Recently, most commercially available image pickup devices are used for taking photographs or recording video images. With increasing development of digital techniques, text documents are gradually created as electronic files. Generally, text documents are scanned by scanning apparatuses and then saved as electronic files such as PDF (Portable Document Format) files. In a case that no scanning apparatus is connected to the computer, the images of simple documents such as single-sided papers or business cards could be captured by the built-in image pickup device of the computer. The images captured by the built-in image pickup device are usually saved as JPEG (Joint Photographic Experts Group) files. Most users prefer using scanning apparatuses to scan the text documents as PDF files because the texts and signs contained in the PDF files are readable by the computer systems and could be converted as editable text files.
As known, optical character recognition (OCR) is a process of capturing an image of a document and then extracting the texts from that image. Recently, it is possible to analyze images that are captured from the image pickup device by the OCR technique and saved as JPEG files. As a consequence, individual texts or signs contained in the JPEG files are recognized and converted as editable text files. Since the associated OCR techniques are well established, the image pickup device is gradually adopted to obtain electronic files of the documents.
When an image pickup device of a notebook computer is used to capture the image of a document, some difficulties possibly occur. For example, the image pickup device of a typical notebook computer has a focal length of approximately 60 centimeters. For using the image pickup device to capture a full-page image of a document that is fully within the shooting range of the image pickup device, all pixels of the image pickup device need to be utilized to obtain the largest image. Under this circumstance, the distance between the image pickup device and the document is deviated from the preset focal length (i.e. 60 centimeters), and thus the obtained image of the document is usually blurred and fails to be recognized by the naked eyes. After the texts or signs contained in the blurred image are subject to an OCR technique, the recognition rate is insufficient.
For using the image pickup device to obtain a sharp image of the document, an external lens is usually arranged in front of the image pickup device. The use of the external lens could change the preset focal length of the image pickup device in order to capture the image of either a nearby document or a faraway object. Generally, the external lens includes a prime lens or a zoom lens. The prime lens is a photographic lens whose focal length is fixed. The zoom lens is a mechanical assembly of lens elements with the ability to vary its focal length. For mounting the external lens in the front of the image pickup device, a fixing device is necessary for connecting the external lens with the image pickup device. Moreover, the fixing device should have the capability of aligning the external lens with the image pickup device in order to enhance the imaging quality of the image pickup device.
FIG. 1 is a schematic side view illustrating a notebook computer having an external lens device mounted thereon according to the prior art. As shown in FIG. 1, the notebook computer 1 comprises a base 10 and an upper cover 11. A keyboard 101 is mounted on the base 10. Via the keyboard 101, the user may input characters or signs. The upper cover 11 comprises a screen 111 and an image pickup device 112. The image pickup device 112 is disposed on an upper edge of the upper cover 11 for capturing an image of a document. The screen 111 is used for showing the operating conditions of the notebook computer 1. In a case that an image is captured by the image pickup device 112, the image is shown on the screen 111. As shown in FIG. 1, the fixing device 12 comprises a hooking element 121, which is attached onto the upper cover 11 of the notebook computer 1. After the hooking element 121 is attached onto the upper cover 11, the fixing device 12 is connected with the notebook computer 1. Since the external lens 13 is connected with the fixing device 12, the external lens 13 will be arranged in front of the image pickup device 112.
As shown in FIG. 1, an example of the external lens 13 is a close-up lens. After the external lens 13 is mounted in front of the image pickup device 112 by the fixing device 12, the preset focal length of the image pickup device 112 is shortened to capture an image of a nearby document through the external lens 13. Conventionally, no proper alignment tool is used for facilitating the user to align the external lens 13 with the lens of the image pickup device 112 after the external lens 13 is mounted in front of the image pickup device 112. If the external lens 13 is not precisely aligned with the lens of the image pickup device 112, the imaging quality of the image pickup device 112 is deteriorated. Conventionally, the user could visually discriminate whether the external lens 13 is aligned with the lens of the image pickup device 112. Since no objective alignment tool is used, the imaging quality of the image pickup device 112 is usually deteriorated.